This invention generally relates to automobile suspension systems. More particularly, it pertains to an improved automobile suspension system with a shock absorber of multi-degree variable-damping-coefficient type between a sprung element and an unsprung element.
An automobile suspension system generally includes between a body (i.e., the sprung element) and wheels (i.e., the unsprung elements) shock absorbers to damp up-and-down vibration of the wheels. Many types of shock absorbers are known in the art. For example, there is known a shock absorber of bi-degree variable-damping-coefficient type so that the damping coefficient can vary from a greater degree to a lesser degree, and vice versa. A shock absorber of multi-degree or infinitely variable-damping-coefficient type is likewise known.
Conventionally, such a shock absorber employs a control method that its damping coefficient is changed and controlled so as to harmonize an actual damping force, generated by the shock absorber, with a target damping force (i.e., the skyhook damper force) that causes no sprung vertical motion. Japanese Patent Application Laying Open Gazette No. 60-248419 discloses a shock absorber control method which checks for the agreement between a sign of the relative displacement between the sprung element and the unsprung element and another sign of the differentiated value thereof, (i.e., the relative velocity between the sprung element and the unsprung element). At the time when these two signs agree with each other, the damping coefficient of the shock absorber increases, which results in the increase of the damping force generated by the shock absorber. On the other hand, at the time when the two signs disagree with each other, the damping coefficient of the shock absorber decreases, which results in the decrease of the damping force generated by the shock absorber.
Although in the foregoing conventional control method the target damping force is so set that it causes no sprung vertical motion, traveling stabilization decreases because the wheels tend to separate from the road surface at an oscillation frequency in the vicinity of the unsprung resonant point.